Use custom digit grouping

clippy emits a bunch of:

 warning: digits grouped inconsistently by underscores

We have a custom grouping elsewhere in this file

  10_000_000_00 sats == 10 BTC

Fix up all instances of large sats amount to uniformly using this format
and add compiler directives where needed to shoosh clippy.

Author: tcharding

src/util/amount.rs

@@ -1622,9 +1622,9 @@ mod tests {
         assert_eq!(p("1.1", btc), Ok(Amount::from_sat(1_100_000_00)));
         assert_eq!(p("100", sat), Ok(Amount::from_sat(100)));
         assert_eq!(p("55", sat), Ok(Amount::from_sat(55)));
-        assert_eq!(p("5500000000000000000", sat), Ok(Amount::from_sat(5_500_000_000_000_000_000)));
+        assert_eq!(p("5500000000000000000", sat), Ok(Amount::from_sat(55_000_000_000_000_000_00)));
         // Should this even pass?
-        assert_eq!(p("5500000000000000000.", sat), Ok(Amount::from_sat(5_500_000_000_000_000_000)));
+        assert_eq!(p("5500000000000000000.", sat), Ok(Amount::from_sat(55_000_000_000_000_000_00)));
         assert_eq!(
             p("12345678901.12345678", btc),
             Ok(Amount::from_sat(12_345_678_901__123_456_78))
@@ -2006,6 +2006,7 @@ mod tests {
 
     #[cfg(feature = "serde")]
     #[test]
+    #[allow(clippy::inconsistent_digit_grouping)] // Group to show 100,000,000 sats per bitcoin.
     fn serde_as_btc() {
         use serde_json;
 
@@ -2041,6 +2042,7 @@ mod tests {
 
     #[cfg(feature = "serde")]
     #[test]
+    #[allow(clippy::inconsistent_digit_grouping)] // Group to show 100,000,000 sats per bitcoin.
     fn serde_as_btc_opt() {
         use serde_json;
 
@@ -2054,8 +2056,8 @@ mod tests {
         }
 
         let with = T {
-            amt: Some(Amount::from_sat(2__500_000_00)),
-            samt: Some(SignedAmount::from_sat(-2__500_000_00)),
+            amt: Some(Amount::from_sat(2_500_000_00)),
+            samt: Some(SignedAmount::from_sat(-2_500_000_00)),
         };
         let without = T {
             amt: None,
@@ -2085,6 +2087,7 @@ mod tests {
 
     #[cfg(feature = "serde")]
     #[test]
+    #[allow(clippy::inconsistent_digit_grouping)] // Group to show 100,000,000 sats per bitcoin.
     fn serde_as_sat_opt() {
         use serde_json;
 
@@ -2098,8 +2101,8 @@ mod tests {
         }
 
         let with = T {
-            amt: Some(Amount::from_sat(2__500_000_00)),
-            samt: Some(SignedAmount::from_sat(-2__500_000_00)),
+            amt: Some(Amount::from_sat(2_500_000_00)),
+            samt: Some(SignedAmount::from_sat(-2_500_000_00)),
         };
         let without = T {
             amt: None,